With the rapid development of information industry, especially increasing requirements from mobile Internet and Internet of things (IoT), mobile communication techniques are facing unprecedented challenges. According to a report of ITU, ITU-R M.[IMT.BEYOND 2020.TRAFFIC], the mobile traffic volume in 2020 is estimated to be almost 1000 times of that in 2010 (which is in the 4G era), and the number of connected user terminals may exceed 17 billion. The number of connected devices will see more drastic growth when a mass of IoT devices are gradually connected to the mobile communication network. In view of the challenge, the fifth generation mobile communication technology (5G) for the 2020 era is being widely studied by the communication industry and the academia. The ITU report ITU-R M.[IMT.VISION] discusses the framework and overall target of 5G, detailing the prospect of demands for 5G, application scenarios, and key parameters. The ITU report ITU-R M.[IMT.FUTURE TECHNOLOGY TRENDS] provides information regarding future trends of the 5G technology, aiming at remarkably increasing system throughput, providing uniform user experiences, improving extensibility to support IoT, reducing time delay, increasing power efficiency, reducing costs, increasing network flexibility, supporting emerging services, improving flexibility in utilizing the spectrum resources and the like.
Waveforms and multiple access techniques are important basis for air-Interface design, and 5G is no exception. At present, orthogonal frequency division multiplexing (OFDM), acting as a typical multi-carrier modulation (MCM) technique, has been widely used in audio/video broadcasting systems and civil communication systems, e.g., long term evolution (LTE) systems corresponding to evolved universal terrestrial radio access (EUTRA) defined by 3rd generation partnership project (3GPP), digital video broadcasting (DVB) and digital audio broadcasting (DAB) in Europe, very-high-bit-rate digital subscriber loop (VDSL), IEEE802.11a/g wireless local area (WLAN), IEEE802.22 wireless regional area network (WRAN), IEEE802.16 world interoperability for microwave access (WiMAX), and so on. OFDM divides a broad band channel into multiple parallel narrow band subchannels/subcarriers to convert transmission of a high rate data flows in a frequency selective channel into transmission of multiple lower rate data flows in multiple parallel flat subchannels. OFDM can greatly improve the anti-multipath interference capabilities of the system. Furthermore, modulation and de-modulation of OFDM can be simplified using inverse fast Fourier transform/fast Fourier transform (IFFT/FFT). In addition, the use of cyclic prefix (CP) converts the linear convolution of a channel into circular convolution. According to characteristics of circular convolution, when the CP length is larger than the maximum multipath time delay in the channel, inter-symbol interference (ISI) can be eliminated simply by using single tap frequency domain channel equalization. The processing complexity of receivers is remarkably reduced. CP-OFDM can generate waveforms satisfying the demands of 4G mobile broadband (MBB) services, but may have insufficiencies in more challenging 5G scenarios, which mainly include (1), the CP for anti-ISI may significantly decrease the spectrum efficiency in 5G low time delay scenarios. Because low time delay transmission may greatly reduce the length of OFDM symbols while the CP length is determined only by the channel impulse response, the ratio of CP length to OFDM symbol length may become very large. The overhead will result in remarkable loss in spectrum efficiency, and it is unacceptable. (2), strict requirement on time synchronization may cause large signaling overhead which is necessary to maintain close-loop synchronization in 5G IoT scenarios. Further, the strict time synchronization scheme may make frame structures less flexible and cannot satisfy different synchronization demands of different types of services. (3) OFDM uses rectangular pulse, leading to slow sidelobe roll-off, which causes large amount of out-of-band emission. Therefore the OFDM is very sensitive to carrier frequency offset. However, 5G requires much of flexible accessing and sharing of fragmented frequency spectrum, but the large amount of out-of-band emission limits the flexibility of spectrum access. In other words, large amount of frequency domain guard band is required, which reduces the utilization of spectrum. The above insufficiencies are mainly resulted from intrinsic properties of the OFDM. Although proper methods can be adopted to mitigate the impact of the insufficiencies, these methods will increase the system design complexity, and the problems cannot be solved fundamentally.
Therefore, according to an ITU report ITU-R M. [IMT.FUTURE TECHNOLOGY TRENDS], a few new waveform techniques (based on MCM) are proposed for 5G. Among them, filtering-based OFDM becomes the focused research object. The F-OFDM introduces time domain filtering based on OFDM. By the design of time domain filter, the F-OFDM can reduce the out-of-band emission caused by time domain rectangular window significantly, and keep the specific advantages of OFDM, which includes the orthogonality of subcarrier in complex field, competing the frequency selective fading by adding CP. Effective constraint of out-of-band emission can support fragmented frequency spectrum well. Compared with other new waveform technology, such as filter-bank multi-carrier (FBMC), the F-OFDM keeps the orthogonality of subcarriers in the complex field, and supports the fading channel and multi-antenna system better. The F-OFDM supports subband filtering, i.e., it divides the available frequency band into non-overlapping subbands, and different multi-carrier modulation parameters, including subcarrier spacing, CP length and so on, can be used for different subbands. In order to prevent the inter-subband interferences, a guard band between subbands may be implemented by inserting subcarriers or not inserting subcarriers, different subbands may be allocated to different services or different users. The subband-based filtering improves the spectrum utilization and the flexibility of spectrum utilization.
The F-OFDM is regarded as one of candidate waveform techniques for 5G for these advantages, but it also has some problems. Specifically, in the F-OFDM, both the transmitter and the receiver need to know information about the filter, so as to compensate for the channel distortion caused by filter. However, the design of time domain filter relates to the subband bandwidth, it is necessary to design different time domain filters for different subband bandwidths, which increases the resources required for storage of the time domain filter and the complexity of channel estimation algorithm, and is unfavorable for the application in low complexity devices in IoT or machine type communication (MTC) scenarios.
In view of the above, in order to make the F-OFDM technique more competitive in 5G candidate techniques, besides exploiting its advantages, the disadvantages need to be solved. With respect to many scenarios especially the narrow-band transmission mode under IoT, it is necessary to solve the problems including high storage requirement and high channel estimation complexity caused by the time domain filter in the F-OFDM.